When functioning properly, the human heart maintains a normal sinus rhythm. Its sinoatrial node generates intrinsic electrical cardiac signals that depolarize the atria, causing atrial contractions. Its atrioventricular node then passes the intrinsic cardiac signal to depolarize the ventricles, causing ventricular contractions.
A normal heart is capable of pumping adequate blood throughout the body's circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Moreover, some patients have poor spatial coordination of heart contractions. For these and other reasons, impaired blood circulation may result. For such patients, a cardiac rhythm management (CRM) system may be used to improve the rhythm or spatial coordination of heart contractions. Such systems often include a CRM device that is implanted in the patient to deliver therapy to the heart.
Cardiac rhythm management systems include, among other things, pacemakers, also referred to as pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pacing pulses, to the heart, such as via an intravascular lead wire or catheter (referred to as a “lead”) having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pacing pulses (this is referred to as “capturing” the heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficacy as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat abnormally slowly. Such pacers may also coordinate atrial and ventricular contractions to improve pumping efficacy.
Cardiac rhythm management systems also include cardiac resynchronization therapy (CRT) devices for spatially coordinating heart depolarizations for improving pumping efficacy. For example, a CRT device may deliver appropriately timed pace pulses to different locations of the same heart chamber to better coordinate the contraction of that heart chamber, or the CRT device may deliver appropriately timed pace pulses to different heart chambers to achieve better synchronization.
Cardiac rhythm management systems also include defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Such defibrillators include cardioverters, which typically synchronize the delivery of such stimuli to sensed intrinsic heart activity signals. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat abnormally quickly. Such too-fast heart rhythms can also cause impaired blood circulation. A defibrillator is capable of delivering a high energy electrical stimulus that is sometimes referred to as a defibrillation countershock, also referred to simply as a “shock.” The shock terminates the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the improved pumping of blood. In addition to pacers, CRT devices, and defibrillators, CRM systems also include CRM devices that combine these functions, as well as monitors, drug delivery devices, and any other implantable or external systems or devices for diagnosing or treating the heart. Cardiac rhythm management systems often include external local or remote user interfaces (sometimes referred to as “programmers” or “patient management systems”) for programming one or more therapy control or other parameters of an implantable cardiac rhythm management device, or for receiving physiological or other data communicated from the implantable cardiac rhythm management device. Accurate measurement of hemodynamic conditions is helpful to developing an effective cardiac rhythm management system.